The renal excretory function of rats was investigated under conditions of reduced extracellular fluid volume (ECV) obtained by peritoneal dialysis with isotonic glucose solution 10% of the body mass and using sodium-deficient diet (consisting of boiled rice) with intact renin-angiotensin system (RAS) and after angiotensin converting enzyme blockade by Captopril. The experiments were made on male Wistar rats placed in metabolic cages. The diuresis, the excretion of sodium, potassium, chlorine and osmotically active substances in spontaneously released urine were tested over a period of 6 hours. Captopril was administered with Alzet osmotic minipumps at 80 micrograms/h rate of infusion (in the experimental animals with peritoneal dialysis) and intraperitoneally 1 mg/kg (in the animals subjected to sodium-deficient diet). Blocking of the converting enzyme with Captopril was found to increase the diuresis, as well as the sodium and total osmotic excretion after peritoneal dialysis and under sodium-deficient regime. Blocking of RAS with Captopril reduced the adaptive possibilities of the organism in the cases of reduced ECV and sodium-deficient diet.